393. UTF-8 Validation

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A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules:

For 1-byte character, the first bit is a 0, followed by its unicode code.
For n-bytes character, the first n-bits are all one’s, the n+1 bit is 0, followed by n-1 bytes with most significant 2 bits being 10.
This is how the UTF-8 encoding would work:

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Char. number range  |        UTF-8 octet sequence
   (hexadecimal)    |              (binary)
--------------------+---------------------------------------------
0000 0000-0000 007F | 0xxxxxxx
0000 0080-0000 07FF | 110xxxxx 10xxxxxx
0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx
0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

Given an array of integers representing the data, return whether it is a valid utf-8 encoding.

Note:
The input is an array of integers. Only the least significant 8 bits of each integer is used to store the data. This means each integer represents only 1 byte of data.

Example 1:

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data = [197, 130, 1], which represents the octet sequence: 11000101 10000010 00000001.
Return true.
It is a valid utf-8 encoding for a 2-bytes character followed by a 1-byte character.

Example 2:

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data = [235, 140, 4], which represented the octet sequence: 11101011 10001100 00000100.
Return false.
The first 3 bits are all one's and the 4th bit is 0 means it is a 3-bytes character.
The next byte is a continuation byte which starts with 10 and that's correct.
But the second continuation byte does not start with 10, so it is invalid.

解法1：

按照规则来。用一个变量存储当前还需要几个连续的以10开头的byte。
要注意的是在设置mask的时候，需要用0B11100000去判断最初的两个数字是否是11

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public class Solution {
	public boolean validUtf8(int[] data) {
		int varCharLeft = 0;
		for (int b: data) {
			if (varCharLeft == 0) {
                // 要差一位来判断到底有几个1在数字里面，不然不知道到底是有几位。
                // 判断到3的原因是unicode最多是4个byte
				if ((b & 0b10000000) == 0)  varCharLeft = 0;
				else if ((b & 0b11100000) == 0b11000000)  varCharLeft = 1;
				else if ((b & 0b11110000) == 0b11100000)  varCharLeft = 2;
				else if ((b & 0b11111000) == 0b11110000)  varCharLeft = 3;
				else return false;
			} else {
				if ((b & 0b11000000) != 0b10000000)  return false;
				varCharLeft--;
			}
		}
		return varCharLeft==0;
	}
}